Photosensitive elements generally consist of a flat substrate, to which at least one, but as a rule several, thin layers have been applied. At least one of these layers is sensitive to light. Other layers, which may or may not be sensitive to light, fulfill diverse auxiliary functions as, for example, protective layers, filter layers, or antihalation layers. Except for special cases, such as vapour-deposited layers, a binder is always required for the production of photographic layers since the binder imparts the necessary cohesion and adhesion function. For conventional photographic elements, which after exposure are processed with aqueous solutions, a hydrophilic binder which is swellable in water is preferred. Gelatin is particularly suitable as such a binder and is generally the principal binder for photosensitive elements. Additionally, gelatin is used in the food and the pharmaceuticals industries for example to form capsules containing medical preparations, and to prepare jellies. For ease in transport and handling, gelatin generally is sold to the photographic, food and pharmaceutical industries in the form of a relatively dry solid, i.e., pellet, flake, particle, granule, etc. containing not more than 10 to 15 percent moisture. The dry gelatin particles are dissolved into a liquid, generally water, to prepare a gelatin solution suitable for use.
Conventional methods used to dissolve gelatin have consisted of methods in which a fixed amount of dry solid particles of gelatin is immersed in a fixed amount of aqueous solution, e.g., water at about 60.degree. to 80.degree. F. (15.5.degree. to 26.7.degree. C.), and generally soaked for a period of time to thoroughly wet and swell the dry particles with the water. Thereafter, the mixture of particles and water mixture is agitated and heated to a temperature and for a time sufficient to dissolve the gelatin particles into solution. There are several problems associated with this cold soaking mixing method for dissolving gelatin. One of the problems is that the solid gelatin particles are not easily wetted and tend to float on the liquid surface. The non-wetting is even more troublesome if the gelatin is added to hot water, i.e., 85.degree. F. (29.4.degree. C.) or higher, or to previously prepared gelatinous solutions. In such cases, the particles become sticky and agglomerate before they can be adequately dispersed, and form large lumps that dissolve very slowly. If, in an effort to improve dissolution, the agitation of the solution is increased, excessive quantities of air are entrained in the solution causing undesirable bubbles and foam. This foam collects at the top surface of the solution stiffening as it dries, and frequently, portions of the stiffened foam fall back into the gelatin solution which do not readily dissolve. Filtration does not always adequately separate these agglomerates and undissolved foam portions from the solution, especially at elevated pressures which can result in `extrusion` of undissolved gelatin through the filter. In the case of photographic materials, these agglomerates and undissolved foam portions adversely affect the coated quality of a gelatin-containing layer.
Furthermore, this method is a time consuming batch process in which a minimum of about 40 to 60 minutes is needed to completely dissolve gelatin particles in the water. Generally, gelatin particles are soaked 10 to 60 minutes, digested or dissolved for at least 15 minutes at an elevated temperature, and there is considerable time required for the mixture in the vessel to reach the elevated temperature as it is dependent upon heat transfer rates, volume of the vessel, and other factors knowledgeable to one skilled in the art. Also, if there are any delays in the consumption of the gelatin solution due to upsets in subsequent process steps, the gelatin solution can readily degrade as the elevated temperature causes the water to evaporate from the solution and other problems can occur, such as bacterial growth, depending upon the additives to the gelatin solution.
It is an object of this invention to provide a method for preparing gelatin solutions in which the gelatin particles are dissolved in an aqueous solution and do not have the dissolution problems associated with prior methods.
It is another object of this invention to provide a method of preparing gelatin solutions in-line which is continuous and is accomplished in a short time period so that subsequent process steps in the formation of photographic elements can receive dissolved gelatin solution on demand, for immediate consumption. These and other objects of the present invention will be clear from the following description.